1. Field of the Invention
The present invention relates to a velocity modulation apparatus for a three-tube projection television (TV), and more specifically to a velocity modulation apparatus for effectively implementing velocity modulation on the basis of human eye's relative spectral luminous efficiency (visibility) characteristics with respect to three primary colors, in a three-tube projection television.
2. Description of the Prior Art
There has been well known a velocity modulation technique such that the electron beam scanning speed can be controlled by modulating an electron beam scanning at a constant scanning speed on the surface of a Braun tube (cathode ray tube), with the use of a coil disposed externally of the electron gun, without controlling the intensity of the electron beam.
Since the picture image is relatively darkened when the electron beam scanning speed is high and relatively brightened when low, it is possible to emphasize the picture image contour. Accordingly, on the basis of the velocity modulation technique for implementing the contour compensation as described above, it is possible to virtually improve the focus status by- sharpening the change in brightness at the edge portions of a picture image.
FIG. 6 shows a velocity modulation (VM) apparatus used for a one-tube projection television provided with a single electron gun of a direct viewing tube (ordinary CTV) type.
The velocity modulation apparatus of this projection television includes a VM signal extracting circuit 1 for extracting a velocity modulation signal by detecting the change of a luminance signal Y, and a driving circuit 10 for driving a VM coil 12 by amplifying the velocity modulation signal outputted from the VM signal extracting circuit 1 at a predetermined amplification factor.
The VM coil 12 is disposed on a Braun tube, and energized in response to the velocity modulation signal. As a result, the horizontal deflection magnetic field is modulated by the magnetic field generated by this VM coil 12, so that a deflection current can be modulated. Owing to this modulation operation, the deflection speed is decelerated (or accelerated) during the leading period of the video signal and accelerated (or decelerated) during the trailing period thereof, so that the electron beam speed can be controlled.
In the above-mentioned modulation operation, when the deflection speed is decelerated, since the luminous time is prolonged, the picture image is brightened; on the other hand, when accelerated, since the luminous time is shortened, the picture image is darkened, so that a sharp image can be obtained.
FIG. 7 is a circuit diagram showing a velocity modulation apparatus for a prior art three-tube color projection television.
In this velocity modulation apparatus for a three-tube projection television, the velocity modulation apparatus for a one-tube projection television shown in FIG. 6 is modified so as to be applied to a velocity modulation apparatus for a three-tube projection television, in which a red color (R) VM coil 7, a green color (G) VM coil 8, and a blue color (B) VM coil 9 are connected in series to each other so as to be driven by a common driving circuit 10.
The red color VM coil 7, the green color VM coil 8 and the green color VM coil 9 are disposed on the corresponding one of the three Braun tubes, respectively so as to provide the contour compensation of picture images on the three Braun tubes, respectively.
In FIG. 7, the number of turns of each of these VM coils 7 to 9 is equal to each other, as shown by the turn ratios of (1):(1):(1) in FIG. 7.
In the velocity modulation apparatus of the three-tube projection television as shown in FIG. 7, since the number of turns of each of the VM coils 7 to 9 is the same and further because these coils 7 to 9 are energized by the common driving circuit 10, the intensity of magnetic field of each of these coils 7 to 9 is equal to each other. Therefore, the three Braun tubes are velocity-modulated under the same conditions.
However, the three primary colors R (red), G (green) and B (blue) are different from each other in the relative visibility (luminous factor) characteristics as shown in FIG. 2, in which the sensitivity of green is the highest; that of red is the medium; and that of blue is the lowest. Therefore, in the case of the velocity modulation technique by which the focus status can be improved virtually by sharpening the change in brightness at the edge portions of a picture image, there exist various problems in that the circuit configuration of the velocity modulation apparatus consumes power wastefully and therefore the efficiency is low, because the three primary colors of R, G and B are velocity-modulated under the same conditions, irrespective of the difference in the human's eye sensitivity among these. The above-mentioned wasteful power consumption inevitably causes a rise in temperature.
Further, being different from the circuit shown in FIG. 7, it may be possible to provide the red color VM coil 7, the green color VM coil 8 and the blue color VM coil 9 each having the same number of turns and connected in parallel to each other so as to be energized, independently by three separate driving circuits the same as the driving circuit 10 as shown in FIG. 7. In this case, however, the same problems as described above will arise.